Methanol synthesis is generally performed by passing a synthesis gas comprising hydrogen, carbon oxides and any inert gases at an elevated temperature and pressure through one or more beds of a methanol synthesis catalyst, which is often a copper-containing composition. Methanol is generally recovered by cooling the product gas stream to below the dew point of the methanol and separating off the product as a liquid. The process is usually operated in a loop: thus the remaining unreacted gas stream is usually recycled to the synthesis reactor as part of the synthesis gas via a circulator. Fresh synthesis gas, termed make-up gas, is compressed and added to the recycled unreacted gas to form the synthesis gas stream. A purge stream is often taken from the circulating gas stream to avoid the build up of inert gases. Such a process is described for example in EP 0329292.
This arrangement is, however, unsuitable for gases whose stoichiometric number (R), defined by the formula;
  R  =                    [                  H          2                ]            -              [                  CO          2                ]                            [                  CO          2                ]            +              [        CO        ]            is less than 2, signifying that the gas is deficient in H2 for the manufacture of methanol. Synthesis gases deficient in hydrogen may be obtained from reforming processes including a step of partial oxidation, such as autothermal reforming. In such a case, the hydrogen will be consumed in the methanol synthesis reaction while a substantial portion of the carbon oxides remain unreacted leading to a composition in the synthesis loop which has very high levels of carbon oxides but is low in hydrogen. This has several consequences, among them that the required catalyst volume will be high and that the level of by-products (higher alcohols and ketones in particular) will be much higher than normal.
It is known that hydrogen can be recovered from the purge gas stream using a hydrogen recovery unit and recycled back into the feed gas so that the gas within the synthesis loop is significantly more H2-rich than is the synthesis gas. However, one of the difficulties with this approach is that for synthesis gases that are very deficient in H2, it is necessary to recover large quantities of H2 from the purge gas, and to have such a large flow of purge gas means either operating the synthesis loop at low pressure or having a low ratio of flow of recycle gas to flow of fresh synthesis gas. Running the synthesis loop at low pressure is not attractive for large commercial scale plants due to the size of the pipework, vessel diameters, etc., whereas running with a low recycle ratio can impose restrictions on the methanol synthesis reactor that may be unacceptable. For instance, a low recycle ratio means that using the circulating gas to cool the reaction, either in a quench cooled or tubular reactor, is impossible, so the only option is a steam-raising reactor. Furthermore, the low recycle ratio means that the reactant concentration at the inlet to the reactor is high as will be the reaction rates, so in order to prevent excessive temperatures in the catalyst bed, the catalyst will have to be installed inside the tubes of a tubular steam raising reactor. This is an unattractive choice as this leads to poor utilisation of the volume within the shell of the reactor as well as the requirement for extremely thick, heavy tube sheets. There is also a limit to the pressure at which steam can be raised, so utilisation of this steam may complicate the design of the steam system on such a plant so increasing cost and reducing operability and reliability.
Another alternative is to take a side-stream of the fresh synthesis gas, also termed make-up gas (MUG), recover hydrogen from it using a hydrogen recovery unit, and feed this hydrogen back into the synthesis gas. However, the drawback to this arrangement is that some hydrogen is lost within the hydrogen recovery unit before it ever gets to the synthesis loop, and the synthesis gas, after enrichment with this hydrogen, will now have a stoichiometry number greater than 2, so the purge gas will now consist of a significant portion of unreacted H2. The effect of this is that the quantity of methanol produced from a fixed quantity of synthesis gas is reduced, and so a large synthesis gas generation unit is required for a given production capacity. Since the synthesis gas generation unit is the most expensive part of the plant, increased spending in this area is uneconomic.
Thus there is a need to provide a methanol synthesis process including a step of hydrogen recovery without the disadvantages of either method.